Bugfix `fabric.Gradient#fromElement`and add more gradient tests

Convert opacity value into float value. If stop-opacity is set as attribute (not as style attribute) `el.getAttribute('stop-opacity')` returns null. Problem exists since commit a0b7fa4eb9a8479165e47394e83100f416b1ddcb.

Add default value rgb(0,0,0) for stop-color and 1 for stop-opacity.
Add more gradient tests.