When building arrowhead URL remove fragment ID
When building arrowhead URL remove fragment ID

We're using a fragment ID to refer to the definition of an
arrowhead. However, if the URL we're using already includes a
fragment, the previous implementation would just just append to the
fragment id instead of replacing it. This patch replaces the entire
fragment.

cpettitt/dagre-d3,cpettitt/dagre-d3,dagrejs/dagre-d3,dagrejs/dagre-d3,cpettitt/dagre-d3