Make parameter filtering truly culture-insensitive

Dapper's parameter filtering is currently case-insensitive, but relies on the current culture (depending on which bit of code you look at). In situations where the current culture is variable, certain cultures may unexpectedly produce exceptions when the parameter name in-query is in a different case than the property of the provided parameter object.

Making the FilterParameters regex RegexOptions.CultureInvariant fixes this, and I removed a seemingly redundant check for the variable presence within the loop that already did the case-insensitive culture-invariant comparison.