If HTTP charset header is empty, do not append content encoding to Content-Type header
Add unit test for Content-Type header with and without charset set
xbuild: include a Microsoft.Portable.Common.targets file (for F#)

F# PCL projects expect this file to exist in:
$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.Common.targets

So in order to avoid pristine F# PCL projects (created from VS) fail
the build that file has to exist, and import the Core targets like
its CSharp counterpart (Microsoft.Portable.CSharp.targets) does.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=17906
BigIngeter: new LeadingZeroCount helper method

- The new helper method LeadingZeroCount returns the number of bits
  that are set to 0 at the start of an uint.
[Performance] BigInteger: cast to double and float

- The new implementation converts directly to double or float.
BigInteger: added tests for double conversion
BigInteger: more test cases for cast to double
[Fix] BigInteger: improved cast to double

- Added overloads for PopulationCount and LeadingZeroCount that take ulong.
- Added helper method BuildDouble that constructs a double by sign, mantissa
  and exponent. This method truncates the mantissa, thus may differ in
  rounding to Math.Pow.
- The new implementation avoids Math.Pow by using BuildDouble.
- Passing all recently added tests.
[Fix] BigInteger: reimplemented cast to single (float)

- The new implementation casts to double and then to float.
Merge pull request #930 from theraot/master

New cast to double and float for BigInteger
Correctly set Count after call to ConcurrentStack.PushRange()
Merge pull request #931 from exony-craigminihan/bugzilla#18097

Correctly set Count after call to ConcurrentStack.PushRange()
[mdoc] Fix the tests.
[corlib] Implement CultureInfo.ClearCachedData. Fixes #18162
Fix the build.
[sdb] Add some debug output to help track down #18100.
[runtime] Implement wapi_process_get_path () on non osx platforms by falling back to get_process_name_from_proc ().
[runtime] Fix a memory leak.
[runtime] Fix a memory leak.
[runtime] Fix a memory leak.