ToolTask UseUtf8Encoding value of 'true' is equal to 'always'

[jrdodds]

Fixes #10907

Context

The UseUtf8Encoding property of the Exec task and other ToolTask derived tasks should accept true and equivalent to always.

Changes Made

ToolTask doesn't validate or interpret the UseUtf8Encoding property. The value of the property is passed to and interpreted by EncodingUtilities.BatchFileEncoding().

EncodingUtilities was modified to add a const string for TRUE and the BatchFileEncoding() method was modified to handle TRUE and ALWAYS the same.

Testing

Created a unit test that compares the returns for ALWAYS and TRUE, confirmed that the unit test failed, modified the code, and confirmed the unit test passes.

Tested on Windows.

Notes

On non-Windows platforms the UseUtf8Encoding property has no effect.

[maridematte]

Looks good to me, we appreciate the fast work on the issue.

[rainersigwald]

Thanks @jrdodds!