Enable and fix lint unnecessary_brace_in_string_interps (#313)

Author: natebosch

analysis_options.yaml

@@ -62,7 +62,7 @@ linter:
   - throw_in_finally
   - type_init_formals
   - unawaited_futures
-  # unnecessary_brace_in_string_interps
+  - unnecessary_brace_in_string_interps
   - unnecessary_const
   - unnecessary_getters_setters
   - unnecessary_lambdas

test/multipart_test.dart

@@ -21,7 +21,7 @@ void main() {
 
   test('boundary characters', () {
     var testBoundary = String.fromCharCodes(BOUNDARY_CHARACTERS);
-    var contentType = MediaType.parse('text/plain; boundary=${testBoundary}');
+    var contentType = MediaType.parse('text/plain; boundary=$testBoundary');
     var boundary = contentType.parameters['boundary'];
     expect(boundary, testBoundary);
   });