Fix incorrect transform of naked pre-existing this (#41)

* Failing test for #30

* Resolve #30

Author: NullVoxPopuli

transforms/no-implicit-this/__testfixtures__/handlebars-with-positional-params.input.hbs

@@ -6,4 +6,5 @@
 {{my-component (my-helper property)}}
 {{my-component (my-helper "string")}}
 {{my-component (my-helper 1)}}
+{{get this 'key'}}
 

transforms/no-implicit-this/__testfixtures__/handlebars-with-positional-params.output.hbs

@@ -6,4 +6,4 @@
 {{my-component (my-helper this.property)}}
 {{my-component (my-helper "string")}}
 {{my-component (my-helper 1)}}
-
+{{get this 'key'}}

transforms/no-implicit-this/helpers/plugin.js

@@ -34,6 +34,7 @@ function transformPlugin(env, runtimeData, options = {}) {
     ElementNode: paramTracker,
     PathExpression(ast) {
       if (ast.data) return;
+      if (ast.original === 'this') return;
 
       let token = ast.parts[0];