Add hint when if or unless is followed by do

[schneems]

A typo in RSpec is typing if instead of it. Depending on the font, they can be hard to distinguish at a glance:

if "does something" do  # Should be `it "does something" do`
  # ...
end

This results in a confusing "Unmatched keyword, missing end'?" error because both ifanddorequire their ownend. And some keywords like whileandlooptakedo, so it's not always obvious that iforunless` don't.

The message needs to accommodate a scenario where it's unknown if the value inside accepts a block like:

if method_call do
end

This might be perfectly valid ruby code, so you wouldn't want to assert "if does not take a do" (since that's not the problem...the problem is that both if and do require an end to close, but the code doesn't provide that.

Before:

Unmatched keyword, missing `end' ?

  2  describe "something" do
> 3    if "does something" do
> 5    end
  6  end

After:

Unmatched keyword, missing `end' ?
Both `if` and `do` require an `end`.

  2  describe "something" do
> 3    if "does something" do
> 5    end
  6  end

Close #206