Batch requests fail if response contains duplicate HTTP headers

[erickoledadevrel]

A BatchRequest will fail if the HTTP responses in the batch contain duplicate HTTP headers, generating a stack trace like the one seen below:

System.AggregateException was unhandled
HResult=-2146233088
Message=One or more errors occurred.
Source=mscorlib
StackTrace:
at System.Threading.Tasks.Task.WaitAll(Task[] tasks, Int32 millisecondsTimeout, CancellationToken cancellationToken)
at System.Threading.Tasks.Task.WaitAll(Task[] tasks, Int32 millisecondsTimeout)
at System.Threading.Tasks.Task.WaitAll(Task[] tasks)
at CalendarQuickstart.Program.Main(String[] args) in c:\Users\ekoleda\Documents\Visual Studio 2013\Projects\ConsoleApplication2\ConsoleApplication2\Program.cs:line 64
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
InnerException: System.ArgumentException
HResult=-2147024809
Message=An item with the same key has already been added.
Source=mscorlib
StackTrace:
at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
at System.Collections.Generic.Dictionary2.Insert(TKey key, TValue value, Boolean add) at System.Collections.Generic.Dictionary2.Add(TKey key, TValue value)
at Google.Apis.Requests.BatchRequest.ParseAsHttpResponse(String content) in c:\code\google.com\google-api-dotnet-client\default\Tools\Google.Apis.Release\bin\Debug\test\default\Src\GoogleApis\Apis\Requests\BatchRequest.cs:line 257
at Google.Apis.Requests.BatchRequest.d__3.MoveNext() in c:\code\google.com\google-api-dotnet-client\default\Tools\Google.Apis.Release\bin\Debug\test\default\Src\GoogleApis\Apis\Requests\BatchRequest.cs:line 204

Duplicate headers are allowed per the HTTP spec, and are equivalent to a single header with a comma-separated list of values:

http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

The source of this exception is due to the fact that a Dictionary is used to hold the header keys and value, which assumes unique keys:

https://github.com/google/google-api-dotnet-client/blob/81354f5b081d46cbb36768d867044d11046afd7f/Src/GoogleApis/Apis/Requests/BatchRequest.cs#L257