Use invariant device locale for number format conversion

[TobiasPott]

This PR adds invariant culture to the conversion methods used in Parse. It prevents conversion based on a device's locale (e.g. french or german) to error on numbers encoded with invariant/US locale (which uses . instead of , as decimal delimiter).

In addition a test method is added (to check this method on the developers machine locale) The test might be extended to explicitly use non-invariant locales for testing (e.g. german or french) to allow it to properly work on machines not using those locales by default. I'm not quite sure on what basis I should pick those locales to test again, except for those two I know of which caused the error described in issue #305. (Feedback is appreciated on this one)

This pull request handles issue #305

[codecov[bot]]

Codecov Report

Merging #308 into master will decrease coverage by 0.31%. The diff coverage is 40%.

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
- Coverage   59.09%   58.77%   -0.32%     
==========================================
  Files          91       91              
  Lines        6126     6128       +2     
==========================================
- Hits         3620     3602      -18     
- Misses       2506     2526      +20

Impacted Files	Coverage Δ
Parse/Internal/ParseCorePlugins.cs	85.31% <ø> (ø)	:arrow_up:
Parse/Public/ParseObject.cs	64.95% <0%> (-0.16%)	:arrow_down:
Parse/Public/ParseClient.cs	74.07% <0%> (ø)	:arrow_up:
Parse/Public/Utilities/Conversion.cs	100% <100%> (ø)	:arrow_up:
Parse/Internal/Utilities/ParseUserExtensions.cs	28.57% <0%> (-42.86%)	:arrow_down:
Parse/Internal/Utilities/ParseQueryExtensions.cs	33.33% <0%> (-33.34%)	:arrow_down:
Parse/Internal/Utilities/ParseObjectExtensions.cs	50% <0%> (-25%)	:arrow_down:

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