Fixed HttpWebRequestTest to compile with mobile NUnit.

Fixed HttpWebRequestTest to compile with mobile NUnit.

Fixes an error in the mono-master-monodroid lane.

[9:24:52] Errors:
[9:24:52]
[9:24:52] /Users/builder/data/lanes/1845/aa3aab5f/source/monodroid/tests/bcl-test/BclTests.sln (default targets) ->
[9:24:52] (Build target) ->
[9:24:52] /Users/builder/data/lanes/1845/aa3aab5f/source/monodroid/tests/bcl-test/BclTests.csproj (default targets) ->
[9:24:52] /Library/Frameworks/Mono.framework/Versions/4.2.0/lib/mono/4.5/Microsoft.CSharp.Targets (CoreCompile target) ->
[9:24:52]
[9:24:52] 	src/System/Test/System.Net/HttpWebRequestTest.cs(2336,13): error CS0117: `NUnit.Framework.Assert' does not contain a definition for `Greater'
[9:24:52]
[9:24:52] 	 13 Warning(s)
[9:24:52] 	 1 Error(s)