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
[llvm] Fix more memory leaks.
[jit] Add support for the new DUMMY_ opcodes to the arm backend.
[llvm] Make the 'llvm_types' hashtable per-domain to fix memory leaks.
[llvm] Remove some debug spew.
Fix the build.
[llvm] Create separate MonoLLVMModule and an LLVM execution engine for each appdomain so they can be freed when the appdomain is unloaded. Part of the fix for #18083.
[runtime] Fix a memory leak.
[runtime] Fix the cross-compiler build.
[corlib] Seal InternalThread
[System.ServiceModel.Web] Fix regression when converting strings using some (like German) cultures [#18131] and add unit test
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
[mcs] Emit correct code for lifted nullable bool logical operators where left operand is not pre cached.
[interpreter] Size reduction
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.