ban-single-arg-parens doesn't work with return type declarations

[felixfbecker]

Bug Report

• TSLint version: 4.4.2
• TypeScript version: 2.2.0
• Running TSLint via: (pick one) anything

TypeScript code being linted

const fn = (param): ReturnType {
  // whatever
}

with tslint.json configuration:

		"arrow-parens": [true, "ban-single-arg-parens"],

Actual behavior

Parentheses are prohibited around the parameter in this single parameter arrow function

If you remove the parenthesis, TypeScript will not be able to compile the file. Parenthesis are required if you declare a return type.

Expected behavior

No error if a return type is declared