Prevent Xamarin Exception on Initialization

[sschaub]

This patch prevents an exception in Xamarin on ParseClient initialization. It provides a proposed fix for #358.

[codecov]

Codecov Report

Merging #359 (a9795bf) into master (f68bde1) will increase coverage by 52.90%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master     #359       +/-   ##
===========================================
+ Coverage        0   52.90%   +52.90%     
===========================================
  Files           0      105      +105     
  Lines           0     4699     +4699     
  Branches        0      830      +830     
===========================================
+ Hits            0     2486     +2486     
- Misses          0     2024     +2024     
- Partials        0      189      +189     

Impacted Files	Coverage Δ
Parse/Infrastructure/HostManifestData.cs	28.57% <0.00%> (ø)
Parse/Platform/Push/ParsePushChannelsController.cs	0.00% <0.00%> (ø)
...rse/Infrastructure/Control/ParseDeleteOperation.cs	83.33% <0.00%> (ø)
Parse/Infrastructure/Data/NoObjectsEncoder.cs	100.00% <0.00%> (ø)
Parse/Infrastructure/Data/ParseObjectCoder.cs	80.85% <0.00%> (ø)
...arse/Infrastructure/Utilities/XamarinAttributes.cs	0.00% <0.00%> (ø)
...rastructure/Utilities/FlexibleDictionaryWrapper.cs	37.14% <0.00%> (ø)
...rm/Installations/ParseInstallationDataFinalizer.cs	0.00% <0.00%> (ø)
Parse/Platform/Users/ParseCurrentUserController.cs	98.50% <0.00%> (ø)
Parse/Utilities/ParseUserExtensions.cs	0.00% <0.00%> (ø)
... and 96 more

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