Cannot assign 'STRING' text to a field.

Steps to reproduce:

1. Create a new Xtext project with the Xtext new project wizard.

2. Modify the grammar to

	grammar org.xtext.example.mydsl.MyDsl with org.eclipse.xtext.common.Terminals

	generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"

	Model:
		a = 'STRING';

3. Execute the GenerateMyDsl.mwe2 workflow.

4. Modify the generated loadModel() test case in the MyDslParsingTest.xtend file to

	@Test
	def void loadModel() {
		val result = parseHelper.parse('''
			STRING
		''')
		Assertions.assertNotNull(result)
		println(result.a)
		val errors = result.eResource.errors
		Assertions.assertTrue(errors.isEmpty, '''Unexpected errors: «errors.join(", ")»''')
	}

5. The test case will fail with the error message: Unexpected errors: XtextSyntaxDiagnostic: null:1 String literal is not properly closed

Start with the grammar shown in the issue, the generated GenerateMyDsl.mwe2 workflow, and MyDslParsingTest.xtend. Run the generated loadModel() test with the literal STRING input and inspect the reported syntax diagnostic. Done means the test parses STRING without the "String literal is not properly closed" error.