fix(filterbox): fixed runtime exception in filterbox for allow multiple selection false

[jitendra-kumawat]

Exception message 
2019-03-11 09:48:21,130:ERROR:root:object of type 'int' has no len()
Traceback (most recent call last):
  File "//projects/open-source/incubator-superset/superset/views/base.py", line 113, in wraps
    return f(self, *args, **kwargs)
  File "/projects/open-source/incubator-superset/superset/views/core.py", line 1251, in explore_json
    samples=samples,
  File "/projects/open-source/incubator-superset/superset/views/core.py", line 1182, in generate_json
    payload = viz_obj.get_payload()
  File "/projects/open-source/incubator-superset/superset/viz.py", line 387, in get_payload
    payload = self.get_df_payload(query_obj)
  File "/projects/open-source/incubator-superset/superset/viz.py", line 402, in get_df_payload
    query_obj = self.query_obj()
  File "/projects/open-source/incubator-superset/superset/viz.py", line 543, in query_obj
    d = super(TableViz, self).query_obj()
  File "/projects/open-source/incubator-superset/superset/viz.py", line 276, in query_obj
    self.process_query_filters()
  File "/projects/open-source/incubator-superset/superset/viz.py", line 270, in process_query_filters
    merge_extra_filters(self.form_data)
  File "/projects/open-source/incubator-superset/superset/utils/core.py", line 811, in merge_extra_filters
    elif filtr['val'] and len(filtr['val']):
TypeError: object of type 'int' has no len()

[codecov-io]

Codecov Report

Merging #7011 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #7011   +/-   ##
=======================================
  Coverage   64.46%   64.46%           
=======================================
  Files         421      421           
  Lines       20521    20521           
  Branches     2245     2245           
=======================================
  Hits        13229    13229           
  Misses       7165     7165           
  Partials      127      127

Impacted Files	Coverage Δ
superset/utils/core.py	88.35% <100%> (ø)	⬆️

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[mistercrunch]

Did a bit of archeology, for reference, this line got added a long time back:
https://github.com/apache/incubator-superset/pull/3668/files#diff-0da28640c22d45c00960bc6d8a4c3483R699

This must be related to Filter Box that don't Allow Multiple:
#6523

All in all, LGTM, it appears the code following what is altered here is assuming the input may or may not be a list

Leaving the PR open so others can intercepts, otherwise will merge in a few days.

[jitendra-kumawat]

@mistercrunch ,can we merge this now.

[jitendra-kumawat]

@mistercrunch did you get time to see this PR. Please share your comments.

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