Remove extra StaticRouter createHref leading slash

[pshrmn]

Fixes #4480 Add a leading slash after concatenating this.props.basename and createURL(path).

// before
addLeadingSlash('') + '/my-path' = '//my-path'
// after
addLeadingSlash('' + '/my-path') = '/my-path'

[timdorr]

Woot!