Default html charset for `stack test --coverage` reports on windows should not be cp437

The charset of of html files produced by stack test --coverage depends on the OS/locale. E.g. on Ubuntu 16 all files will be marked with charset=UTF-8, while on a Win10 machine they will be charset=CP437 (which mangles any character outside this charset; editing manually the charset to utf8 fixes the problem). I did not find any settings that would override the charset/provide custom css/etc.

Steps to reproduce

1. Take any project with a non-empty test suite
2. Cover in that test suite a file with non-ascii characters inside
3. Use Win10
4. Run stack test --coverage
5. Open the produced html report for that file
6. The charset would be set to CP437, making non-ascii part of the source code non-readable

Expected

A setting in a config file/cli option that would put the charset of my choice; or at least charset=UTF-8 everywhere regardless of the OS.

Actual

charset=CP437 where there is no reason to put this charset

Stack version

1.5.1 (latest after stack upgrade); OS=Win10

Method of installation

• Official binary, downloaded from stackage.org or fpcomplete's package repository