Do not change properties in bbox directly
Do not change properties in bbox directly

It is reported in #106 that IE throws if attempting to change properties
on the value returned from getBBox(). Almost certainly these properties
are frozen. This change copies the properties from bbox to a new
unfrozen object.

dominiek/dagre-d3,cpettitt/dagre-d3,cbfx/dagre-d3,cbfx/dagre-d3,dagrejs/dagre-d3,ENCODE-DCC/dagre-d3,cpettitt/dagre-d3,cpettitt/dagre-d3,git-ashish/dagre-d3,9n/dagre-d3,dominiek/dagre-d3,9n/dagre-d3,dominiek/dagre-d3,git-ashish/dagre-d3,9n/dagre-d3,cbfx/dagre-d3,git-ashish/dagre-d3,dagrejs/dagre-d3,ENCODE-DCC/dagre-d3