[FIX][System.Web] Do not crash on duplicate keys in browser capabilities

Correct https://bugzilla.xamarin.com/show_bug.cgi?id=14720 and
probably https://bugzilla.xamarin.com/show_bug.cgi?id=13014

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Signed-off-by: Frederic Mestayer <f.mestayer@fiducial.net>
Add Compat.browser files for net_4_0 and net_4_5

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
[System.Web] Re-enable AppBrowsersTest.cs tests for net_2_0

For now mcs/class/System.Web/Test/mainsoft/NunitWebResources/adapters.browser
depends on System.Web_test_net_2_0.dll

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>