Avoid mutating strings returned by RDF::Turtle::Writer

[masao]

Summary

• Avoid mutating strings returned by RDF::Turtle::Writer in format_turtle
• Prevent previous blank node values from leaking into subsequent Turtle output-
• Add a regression test to ensure formatted URI strings are not mutated

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation or other updates (if none of the other choices apply)