Multiple values passed to WithProperty are overridden

### What You Are Seeing?

MSBuildSettingsExtensions​.WithProperty(MSBuildSettings, ​string, ​String[]) does not work as intended for multiple values of some properties (Maybe all, I don't know). Each value seems to overwrite, so only the last supplied parameter will be applied.

Calling .WithProperty("WarningsNotAsErrors", "1573", "1591") results in /warnaserror-:1591

It looks like method which is used to parse the property dictionary inside MSBuildRunner:

```
private static IEnumerable GetPropertyArguments(IDictionary> properties)
{
foreach (var propertyKey in properties.Keys)
{
foreach (var propertyValue in properties[propertyKey])
{
yield return string.Concat("/p:", propertyKey, "=", propertyValue);
}
}
}
```

Will return "/p:WarningsNotAsErrors=1573", "/p:WarningsNotAsErrors=1591". Which may override when applied sequentially.

Possibly the method could be amended to

```
private static IEnumerable GetPropertyArguments(IDictionary> properties)
{
foreach (var propertyKey in properties.Keys)
{
var values = string.Join(",", properties[propertyKey]);
yield return string.Concat("/p:", propertyKey, "=", values);
}
}
```

Which would return "/p:WarningsNotAsErrors=1573,1591". I'm not sure how this would affect other properties that are used, if a switch must be declared multiple times with single values it would break that.
### What is Expected?

Calling .WithProperty("WarningsNotAsErrors", "1573", "1591") would then become /warnaserror-:1573,1591 when run.

### What version of Cake are you using?

0.14.0.0
### Are you running on a 32 or 64 bit system?

64
### What environment are you running on? Windows? Linux? Mac?

Windows 7
### Are you running on a CI Server? If so, which one?

No, running from powershell script locally
### How Did You Get This To Happen? (Steps to Reproduce)

Add .WithProperty("WarningsNotAsErrors", "1573", "1591") to any MSBuildSettings.

Run in Diagnostic.

Check the value sent to CoreCompile vs the Executing line.
### Output Log

Run with -Mono and -Experimental:

[log.txt](https://github.com/cake-build/cake/files/380631/log.txt)