save as html5 canvas fails (0.92.4)
#### Steps to reproduce:
<!-- Describe what you did (step-by-step) so we can reproduce: -->
- open Inkscape
- create an object, e.g. a box
- save as HTML5 canvas (.html)
#### What happened?
Dialog box appears:
Inkscape has received additional data from the script executed. The script did not return an error, but this may indicate the results will not be as expected.
```
Traceback (most recent call last):
File "ink2canvas.py", line 91, in <module>
ink.affect()
File "inkex.py", line 283, in affect
self.effect()
File "ink2canvas.py", line 86, in effect
self.walk_tree(svg_root)
File "ink2canvas.py", line 77, in walk_tree
self.walk_tree(node)
File "ink2canvas.py", line 73, in walk_tree
if elem.has_gradient():
File "ink2canvas/svg.py", line 102, in has_gradient
style = self.get_style()
File "ink2canvas/svg.py", line 75, in get_style
style = dict([(str.strip(k), str.strip(v)) for k,v in style.items()])
TypeError: descriptor 'strip' requires a 'str' object but received a 'unicode'
```
#### What should have happened?
creating a HTML5-file
#### Inkscape Version and Operating System:
- Inkscape Version: Inkscape 0.92.4 (5da689c313, 2019-01-14)
- Operating System: Windows
- Operating System version: 8.1
issue