WriteOnlyWhenDifferent should check with the same encoding

[stan-sz]

The encoding is missing on the check for WriteOnlyWhenDifferent

msbuild/src/Tasks/FileIO/WriteLinesToFile.cs

Line 102 in 56087a9

string existingContents = System.IO.File.ReadAllText(File.ItemSpec);

while it's present on the subsequent write:

msbuild/src/Tasks/FileIO/WriteLinesToFile.cs

Line 121 in 56087a9

System.IO.File.WriteAllText(File.ItemSpec, contentsAsString, encoding);

[stan-sz]

Mildly related to #3597.
A fix would require capturing this within a UT.