Make has_expected_db_index? check optional on HaveSecureTokenMatcher

[nikosmichas]

The check here could be optional

shoulda-matchers/lib/shoulda/matchers/active_record/have_secure_token_matcher.rb

Lines 78 to 80 in c0960bd

	if !has_expected_db_index?
	@errors << "missing unique index for #{table_and_column}"
	end

has_secure_token encourages to use an index but does not enforce it.

https://github.com/rails/rails/blob/a273da7619ac6a2b2f98532a5610238c68ad219b/activerecord/lib/active_record/secure_token.rb#L31