parse base64 arcgis server responses too

[jgravois]

resolves #782

this PR introduces additional logic to ensure that we parse/display both href and base64 imageData content in ArcGIS Server Map Service Export responses.

the map service @thebasa originally reported a problem with no longer returns MIME data, but luckily another map service on their ArcGIS Server instance does.

var map = L.map('map').setView([45.5, -122.7], 12);
L.esri.basemapLayer('Gray').addTo(map);

L.esri.dynamicMapLayer({
  url: 'https://www.portlandmaps.com/arcgis/rest/services/Public/Auditor_IPR/MapServer',
  layers: [0,3,6]
}).addTo(map);

map service that returns href
map service that returns imageData