added scan string type

[j-forster]

Description

The changes cover multiple issues:
#366
#407
jmoiron/sqlx#349

The ScanType for VARCHAR, TEXT, TINYTEXT and other string-like types correctly scans to go internal string type.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

Both of VARBINARY and VARCHAR uses VARCHAR field type.
This case statement should be merged in the next case.

[methane]

See https://dev.mysql.com/doc/refman/8.0/en/c-api-data-structures.html

To distinguish between binary and nonbinary data for string data types, check whether the charsetnr value is 63.

So correct code is this:

// charsetnr == 63 means this column is binary.
// https://dev.mysql.com/doc/refman/8.0/en/c-api-data-structures.html
if mf.charSet == 63 {
    return scanTypeRawBytes
}
return scanTypeString

[j-forster]

I added the changes that @methane noted.
The tests have been updated to reflect scanTypeString and scanTypeNullString.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

Would you merge master branch?
I think the Travis fail was fixed in master branch already.

[julienschmidt]

Uuuhm. This is a breaking change!
We're not going to do that right before a release.