Removed UNORDERED from StringCollector.characteristics

Intuitively StringCollector should preserve existing encounter order
when used on ordered stream. Thus UNORDERED flag should be omitted form
collector characteristics.
Fixed StringCollector to pass associativity constraint

Collector should pass associativity constraint (i.e. splitting the
computation must produce an equivalent result). Previous implementation
of StringCollector was failing to pass this constraint. It resulted in
incorrect StringCollector behaviour when used with parallel streams.

Added unit tests to check StringCollector behaviour (and prove that
previous implementation was incorrect).
Merge pull request #3 from algirdasrascius/fix_StringCollector_combiner

Fixed StringCollector to pass associativity constraint