Stray comma in map diffs

[michalmuskala]

Environment

• Elixir version (elixir -v): master
• Operating system: OSX

Current behavior

With an assertion like assert %{a: 1} == %{b: 1} ExUnit gives following diff (with proper colours):

Assertion with == failed
code:  %{a: 1} == %{b: 1}
left:  %{a: 1}
right: %{, b: 1}

### Expected behavior

The initial comma in right should be ommited

Assertion with == failed
code:  %{a: 1} == %{b: 1}
left:  %{a: 1}
right: %{b: 1}