Added test for case insensitive field criterion
Make field matches in PostGreSQL case-in-sensitive

Potential issues with this solution:

1) This might be a performance drawback, since all column values must be
   converted to lowercase on each query.

2) PHP and the respective database might have different ideas on how to
   lowercase certain non-ASCII characters. This could lead to issues with
   certain multi-byte encodings in some languages.
Do not lowercase database values

The values are, at least with the current fixtures, already lowercased.
Once indexing is implemented in the legacy backend the lowercasing
should happen there. The old legacy implementation seems to already
implement the lowercasing:
https://github.com/ezsystems/ezpublish-legacy/blob/master/kernel/classes/datatypes/ezstring/ezstringtype.php#L374
on index creation.
Implemented: transformByGroup()

This feature is used to, for example, lowercase random strings in the
ezpublish-legacy, and we need this right now, too.
Use transformation processor to lowercase strings

The same method is used in the legacy implementation to lowercase the
database values. Thus this should be safe and work around issues with
libc implementation of strtolower().
Fixed criterion handler construction in test
Merge pull request #495 from ezsystems/field-case-insensitivity-raw

Fix EZP-20851: Uniform search casing for better pgsql support