Added Webservice tests with REST api.

[304NotModified]

Added units tests for WebserviceTarget with REST api.

• httpGet isn't working and throwing an exception. I will create an issue for that. It is skipped for now.
  Error:

 Result Message:    System.Net.ProtocolViolationException : Cannot send a content-body with this verb-type.
 Result StackTrace: 
 at System.Net.HttpWebRequest.CheckProtocol(Boolean onRequestStream)
    at System.Net.HttpWebRequest.BeginGetRequestStream(AsyncCallback callback, Object state)
    at NLog.Targets.WebServiceTarget.<>c__DisplayClass1.<DoInvoke>b__0(AsyncCallback r) in NLog\src\NLog\Targets\WebServiceTarget.cs:line 151
    at NLog.Targets.WebServiceTarget.DoInvoke(Object[] parameters, AsyncContinuation continuation, HttpWebRequest request, Func`2 beginFunc, Func`2 getStreamFunc) in NLog\src\NLog\Targets\WebServiceTarget.cs:line 218
    at NLog.Targets.WebServiceTarget.DoInvoke(Object[] parameters, AsyncContinuation continuation) in NLog\src\NLog\Targets\WebServiceTarget.cs:line 154
    at NLog.Targets.MethodCallTargetBase.Write(AsyncLogEventInfo logEvent) in NLog\src\NLog\Targets\MethodCallTargetBase.cs:line 79
    at NLog.Targets.Target.WriteAsyncLogEvent(AsyncLogEventInfo logEvent) in NLog\src\NLog\Targets\Target.cs:line 215
    at NLog.LoggerImpl.WriteToTargetWithFilterChain(TargetWithFilterChain targetListHead, LogEventInfo logEvent, AsyncContinuation onException) in NLog\src\NLog\LoggerImpl.cs:line 194
    at NLog.LoggerImpl.Write(Type loggerType, TargetWithFilterChain targets, LogEventInfo logEvent, LogFactory factory) in NLog\src\NLog\LoggerImpl.cs:line 97
    at NLog.Logger.WriteToTargets(LogLevel level, IFormatProvider formatProvider, String message) in NLog\src\NLog\Logger.cs:line 480
    at NLog.Logger.Info(String message) in NLog\src\NLog\Logger1.cs:line 636
    at NLog.UnitTests.Targets.WebServiceTargetTests.<>c__DisplayClassc.<TestWebserviceCall_get>b__b() in NLog\tests\NLog.UnitTests\Targets\WebServiceTargetTests.cs:line 326
    at NLog.UnitTests.Targets.WebServiceTargetTests.StartOwinTest(Action tests) in NLog\tests\NLog.UnitTests\Targets\WebServiceTargetTests.cs:line 422
    at NLog.UnitTests.Targets.WebServiceTargetTests.TestWebserviceCall_get() in NLog\tests\NLog.UnitTests\Targets\WebServiceTargetTests.cs:line 323

• httpPost is only working with [FromBody] which is, ..., akward. Has to do with the "application/x-www-form-urlencoded". Not a lot of people uses "application/x-www-form-urlencoded" with POST to REST.
• .Net 4.5 Only

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This change is