Don't mutate the query builder when calling the statement methods

[bnjamin]

Fixes #712

Clone the queries, so the subsequent calls to #statement and #statement_for_update returns the same result.

avram/src/avram/queryable.cr

Lines 227 to 230 in e638ffb

	def select_count : Int64
	table = "(#{query.statement}) AS temp"
	new_query = Avram::QueryBuilder.new(table).select_count
	result = database.scalar new_query.statement, args: query.args, queryable: schema_class.name

[jwoertink]

nice! Thanks for fixing this 🎉