endsWith and startsWith don't work correctly for some collations

CollationSupport.EndsWIth and CollationSupport.StartsWith use CollationAwareUTF8String.matchAt, which operates byte offsets to compare prefixes/suffixes. This is not correct, since sometimes string parts (suffix/prefix) of different lengths are actually equal in context of case-insensitive and lower-case collations.

Example test cases that highlight the problem:

- assertContains("The İo", "i̇o", "UNICODE_CI", true); for CollationSupportSuite.testContains. 
- assertEndsWith("The İo", "i̇o", "UNICODE_CI", true); for CollationSupportSuite.testEndsWith.

The first passes, since it uses StringSearch directly, the second one does not.