SetYAML(...) ignored for quoted or block strings

The `SetYAML(tag string, value interface{}) bool` method is ignored when the yaml value for the field is either a quoted or block string. Not sure if this is by design...?

Given the following yaml:

    a: not ignored
    b: 'is ignored'
    c: >
      is ignored

If a, b and c are of the same type and the type implements the `SetYAML(...)` method, the method will be ignored for b and c.

I've attached a code sample that shows the issue.

FWIW, changing [this code block][1] so it always executes the `else` block works around the issue.

[1]:http://bazaar.launchpad.net/~gophers/goyaml/trunk/view/head:/decode.go#L300