Don't encode controls as block strings

Fixes #39.

String containing control sequences should be encoded as simple strings
even if they contain newlines, since the block strings can contain only
SourceCharacters.

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to
 ## [Unreleased]
 ### Fixed
 - Result of null encoding
+- Block strings encoding
+- Result of tab and newline encoding
 
 ### Added
 - AST for the GraphQL schema.