Explicit block arguments / fix for methods without arguments

Without changes proposed with this pull request, rdoc failed to handle method arguments properly for the following cases.

Coverage for
```ruby
# test method, yields +foo+
def test_method1
  yield(foo)
end
```
reported undocumented '++' parameter (empty parameter name!)

```ruby
# test method with explicit +block+ argument
def test_method3 &block
  block.call
end
```
reported undocumented '+&block+' parameter, and there were no way to describe it, as parameter names should start with letter, according to rdoc grammar.

```ruby
# only +baz+ has to be documented here
def test_method4 &block
  yield baz
end
```
reported undocumented '+&block+' parameter, and it shouldn't be documented at all.