routePrefix ignored when using ConfigurationBuilder

In `WebJobsStartup`, if merging original configuration with custom configuration, the `routePrefix` defined in `host.json` is ignored and all `HttpTrigger` functions use "api" as `routePrefix`

I have not deployed this to any Azure resource yet, and I'm only running locally, so please bear with the lack of investigative information

#### Investigative information

Please provide the following:

- Timestamp: 2019-04-30
- Function App version: 2.0
- Function App name: N/A
- Function name(s) (as appropriate): N/A
- Invocation ID: N/A
- Region: N/A

#### Repro steps

Use the following code in a WebJobsStartup to add support for custom config-files

```csharp
[assembly: WebJobsStartup(typeof(Startup))]
namespace MyFuncApp {
public class Startup : IWebJobsStartup {
public void Configure(IWebJobsBuilder builder)
{
//Get the current config and merge it into a new ConfigurationBuilder to keep the old settings
ConfigurationBuilder configBuilder = new ConfigurationBuilder();
var descriptor = builder.Services.FirstOrDefault(d => d.ServiceType == typeof(IConfiguration));
if (descriptor?.ImplementationInstance is IConfigurationRoot configuration)
{
configBuilder.AddConfiguration(configuration);
}

IConfigurationRoot config = configBuilder.Build();

//replace the existing config with the new one
builder.Services.Replace(ServiceDescriptor.Singleton(typeof(IConfiguration), config));
}
}
```

This should use the existing configurations inside a new `ConfigurationBuilder`. I could now add `.AddJsonFile(...)` if I wanted to, however for this example I've just created a new `ConfigurationBuilder`, copied the existing config into this builder, created a new config object from this builder, and replace the old config object with the new one, so there are no custom configurations to override anything.

If I debug I can see that the `config` variable contains a (private) `ChainedConfigurationProvider` that contains all the default configurations, including the `HostJsonFileConfigurationProvider` that has `routePrefix: ""`.

When starting the Function app locally I can see that the `routePrefix` is now ignored, and all my functions have "api" at the start of their route, even though my host.json has a blank value. Here's my host.json:

```json
{
"version": "2.0",
"extensions": {
"http": {
"routePrefix": ""
}
}
}
```

It all boils down to this line of code:

```csharp
builder.Services.Replace(ServiceDescriptor.Singleton(typeof(IConfiguration), config));
```

If I remove this the `routePrefix` is blank (as stated in the host.json file). If this line is used the `routePrefix` is "api".

It would therefore seem that there might be a bug in the `routePrefix` resolver that fails to read the prefix from `host.json` whenever a custom `IConfiguration` is used, even if this configuration should have merged in the settings from the old configuration.

#### Expected behavior

Configurations should be merged without overwriting `routePrefix` functionality

#### Actual behavior

`routePrefix` gets reset to `"api"` even if it is `""` in host.json

#### Known workarounds

N/A

#### Related information

* Obviously coded in C#
* HttpBindings