Function declaration returns the function, it should return undefined

[HalidOdat]

Describe the bug
A clear and concise description of what the bug is.

Function declaration returns the function, it should return undefined

To Reproduce

function x() {}

Actual behavior
Returns the function, as if it was a function expression.

Output

{
name: x,
prototype: {
__proto__: {
hasOwnProperty: {
          length: 0,
      extensible: true
            },
    toString: {
          length: 0,
      extensible: true
            },
 constructor: {
  setPrototypeOf: {
              length: 2,
          extensible: true
                },
       prototype: [Cycle],
            name: Object,
          length: 1,
  defineProperty: {
              length: 3,
          extensible: true
                },
  getPrototypeOf: {
              length: 1,
          extensible: true
                },
       __proto__: undefined,
      extensible: true
            },
  extensible: true
        },
extensible: true
    },
length: 0,
__proto__: {
constructor: {
        name: Function,
   prototype: [Cycle],
      length: 1,
   __proto__: undefined,
  extensible: true
        },
__proto__: {
hasOwnProperty: {
          length: 0,
      extensible: true
            },
    toString: {
          length: 0,
      extensible: true
            },
 constructor: {
  setPrototypeOf: {
              length: 2,
          extensible: true
                },
       prototype: [Cycle],
            name: Object,
          length: 1,
  defineProperty: {
              length: 3,
          extensible: true
                },
  getPrototypeOf: {
              length: 1,
          extensible: true
                },
       __proto__: undefined,
      extensible: true
            },
  extensible: true
        },
extensible: true
    },
extensible: true
}

Expected behavior
Explain what you expected to happen, and what is happening instead.

undefined

Build environment (please complete the following information):

• OS: Manjaro Linux
• Target triple: x86_64-unknown-linux-gnu
• Rustc version: rustc 1.43.1 (8d69840ab 2020-05-04)

Additional context
Add any other context about the problem here.

Changing the line https://github.com/boa-dev/boa/blob/master/boa/src/exec/declaration/mod.rs#L38 to return undefined should fix the problem.