Add max zoom default

[ahuang11]

Closes #744

• [] After this gets merged, update Add changelog #741

Unfortunately, I couldn't target OSM specifically; all the combinations I tried:

options.TilePlot.OSM = Options('style', max_zoom=10)
options.TilePlot.Osm = Options('style', max_zoom=10)
options.WMTS.Osm = Options('style', max_zoom=10)
options.WMTS.OSM = Options('style', max_zoom=10)
options.Wmts.OSM = Options('style', max_zoom=10)
options.Wmts.Osm = Options('style', max_zoom=10)
options.TileSources.OSM = Options('style', max_zoom=10)
options.Tile_Sources.OSM = Options('style', max_zoom=10)
options.Tile_Sources.OSM = Options('style', max_zoom=10)
options.TileSources.Osm = Options('style', max_zoom=10)

[hoxbro]

To only change max_zoom of OSM, we can do something like this:

diff --git a/geoviews/plotting/bokeh/__init__.py b/geoviews/plotting/bokeh/__init__.py
index 6a2c16c..6956152 100644
--- a/geoviews/plotting/bokeh/__init__.py
+++ b/geoviews/plotting/bokeh/__init__.py
@@ -49,7 +49,7 @@ from ...operation import (
     project_quadmesh,
     project_vectorfield,
 )
-from ...tile_sources import _ATTRIBUTIONS
+from ...tile_sources import _ATTRIBUTIONS, OSM
 from ...util import line_types, poly_types
 from . import callbacks  # noqa
 from .plot import GeoOverlayPlot, GeoPlot
@@ -60,6 +60,9 @@ except ImportError:
     class ImageStackPlot:
         ...
 
+_TILE_MAX_ZOOM = {OSM: 19}
+
+
 class TilePlot(GeoPlot):
 
     style_opts = [
@@ -102,6 +105,8 @@ class TilePlot(GeoPlot):
                                 'Must contain {X}/{Y}/{Z}, {XMIN}/{XMAX}/{YMIN}/{YMAX} '
                                 'or {Q} template strings.')
             params = {'url': element.data}
+            if element in _TILE_MAX_ZOOM:
+                params['max_zoom'] = _TILE_MAX_ZOOM[element]
             for zoom in ('min_zoom', 'max_zoom'):
                 if zoom in style:
                     params[zoom] = style[zoom]

[ahuang11]

Test will fail due to rc mismatch, but tested the new test and it's ready to merge (I don't have permissions) and added to the changelog.