Make all functions use the default proxy(if the proxy was enabled) when no proxy specified

Expected Behavior

register the ps repository successfully

Current Behavior

nothing happened

Possible Solution

when no proxy specified, then invoke the default proxy (when the proxy was enabled)
https://stackoverflow.com/questions/571429/powershell-web-requests-and-proxies

Steps to Reproduce (for bugs)

set the proxy on computer

Context

related to https://github.com/PowerShell/WindowsCompatibility/issues/45
I want to register ps repository

Your Environment

> $PSVersionTable
Name                           Value
----                           -----
PSVersion                      6.1.0
PSEdition                      Core
GitCommitId                    6.1.0
OS                             Microsoft Windows 10.0.16299
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0
> Get-Module
ModuleType Version    Name                                ExportedCommands
---------- -------    ----                                ----------------
Script     0.0        build                               {Clear-PSRepo, Compress-TestContent, Convert-TxtResourceToXml, ConvertFrom-PesterLog...}
Script     0.0        chuck                               Reset-FolderTime
Script     0.0        Get-PSGalleryApiAvailability
Manifest   6.1.0.0    Microsoft.PowerShell.Management     {Add-Content, Clear-Content, Clear-Item, Clear-ItemProperty...}
Manifest   6.1.0.0    Microsoft.PowerShell.Management     {Add-Content, Clear-Content, Clear-Item, Clear-ItemProperty...}
Manifest   6.1.0.0    Microsoft.PowerShell.Security       {ConvertFrom-SecureString, ConvertTo-SecureString, Get-Acl, Get-AuthenticodeSignature...}
Manifest   6.1.0.0    Microsoft.PowerShell.Utility        {Add-Member, Add-Type, Clear-Variable, Compare-Object...}
Manifest   6.1.0.0    Microsoft.WSMan.Management          {Connect-WSMan, Disable-WSManCredSSP, Disconnect-WSMan, Enable-WSManCredSSP...}
Script     1.1.7.2    PackageManagement                   {Find-Package, Find-PackageProvider, Get-Package, Get-PackageProvider...}
Script     1.0.0.0    posh-git                            {Add-PoshGitToProfile, Expand-GitCommand, Format-GitBranchName, Get-GitBranchStatusColor...}
Script     2.0.1      PowerShellGet                       {Find-Command, Find-DscResource, Find-Module, Find-RoleCapability...}
Script     2.0.0      PSReadLine                          {Get-PSReadLineKeyHandler, Get-PSReadLineOption, Remove-PSReadLineKeyHandler, Set-PSReadLineKeyHandler...}
> Get-Module -ListAvailable PowerShellGet,PackageManagement
ModuleType Version    Name                                PSEdition ExportedCommands
---------- -------    ----                                --------- ----------------
Script     1.1.7.2    PackageManagement                   Desk      {Find-Package, Get-Package, Get-PackageProvider, Get-PackageSource...}
Script     1.6.7      PowerShellGet                       Desk      {Find-Command, Find-DSCResource, Find-Module, Find-RoleCapability...}
> Get-PackageProvider
Name                     Version          DynamicOptions
----                     -------          --------------
NuGet                    2.8.5.210        Destination, ExcludeVersion, Scope, SkipDependencies, Headers, FilterOnTag, Contains, AllowPrereleaseVersions, ConfigFile, SkipValidate
PowerShellGet            1.6.7.0          PackageManagementProvider, Type, Scope, AllowClobber, SkipPublisherCheck, InstallUpdate, NoPathUpdate, AllowPrereleaseVersions, Filter, Tag, Includes, DscResource, RoleCapability, Command, Ac...
> Get-PackageProvider -ListAvailable
Name                     Version          DynamicOptions
----                     -------          --------------
NuGet                    2.8.5.210        Destination, ExcludeVersion, Scope, SkipDependencies, Headers, FilterOnTag, Contains, AllowPrereleaseVersions, ConfigFile, SkipValidate
PowerShellGet            1.6.7.0          PackageManagementProvider, Type, Scope, AllowClobber, SkipPublisherCheck, InstallUpdate, NoPathUpdate, AllowPrereleaseVersions, Filter, Tag, Includes, DscResource, RoleCapability, Command, Ac...

Start by reproducing register ps repository with a system proxy enabled and no explicit proxy, using the PowerShellGet and PackageManagement versions shown. Trace the registration request and related functions to identify where proxy handling diverges. Done means repository registration succeeds through the enabled default proxy when no proxy argument is supplied, with coverage for the affected behavior.