Distinguish between binary and non-binary strings in MySQL

[MaxGabriel]

Closes #448

This PR adds a failing test case demonstrating persistent-mysql writing a PersistText but reading a PersistByteString. The second commit of the PR uses character set information to distinguish between TEXT and BLOB binary and non-binary strings in MySQL, fixing the failing test.

This PR could break existing code. If code is relying on reading a PersistByteString from a MySQL TEXT column, that code will break (as it did for getColumn).

[MaxGabriel]

I realized this also applies to the other string types (not just TEXT and BLOB) and updated the PR accordingly.

[creichert]

i'm not familiar with MySQL specifics but this looks good overall. Are you going to bump the major version?

[MaxGabriel]

@creichert Yeah this probably should have a major version bump. But LTS 3 just started; should we wait awhile to merge this? I can make a PR to yesod-text-markdown to accept either PersistText or PersistByteString to fix the immediate issues people are reporting on StackOverflow.

[gregwebs]

I think it should be merged whenever it is ready. We can create maintenance branches as needed.

[MaxGabriel]

Forgot about this for a few days; I updated the changelog and cabal file. Will merge it in shortly.

[MaxGabriel]

Thanks for fixing build times @snoyberg; these tests were failing on > 50 minute builds before.