Add support for IQueryable.Count(Func<T, bool> predicate)

[sflanker]

Currently .Where(predicate).Count() works but .Count(predicate) does not.

See test case:

Invio.QueryProvider.MySql/test/Invio.QueryProvider.MySql.Test/MySqlQueryableTest.cs

Lines 75 to 81 in 03146ab

	[Fact]
	public override void Count_Filtered() {
	// TODO: Implement support for Count w/ predicate
	var result = this.Suppliers.Where(s => s.Country == "USA").Count();
	Assert.Equal(4, result);
	}