BUG: SparseSeries.value_counts ignores fill_value
- [x] closes #6749
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Also fixed an issue which categorical `value_counts` resets name.

Found current master doesn't work for datetimetz. Let me check.

```
values = [pd.Timestamp('2011-01-01 09:00', tz='US/Eastern'),
                  pd.Timestamp('2011-01-01 10:00', tz='US/Eastern'),
                  pd.Timestamp('2011-01-01 11:00', tz='US/Eastern'),
                  pd.Timestamp('2011-01-01 09:00', tz='US/Eastern'),
                  pd.Timestamp('2011-01-01 09:00', tz='US/Eastern'),
                  pd.Timestamp('2011-01-01 11:00', tz='US/Eastern')]
s = pd.Series(values, name='xxx')

# NG, index must have tz
s.value_counts()
# 2011-01-01 14:00:00    3
# 2011-01-01 16:00:00    2
# 2011-01-01 15:00:00    1
# Name: xxx, dtype: int64
```
 Additionally fixing the other `Categorical` issue.

```
pd.CategoricalIndex([1, 2, 3, 1, 2, 3]).value_counts(normalize=True)
# UnboundLocalError: local variable 'counts' referenced before assignment
```
 @sinhrks pls rebase when you can
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/12835?src=pr) is **84.04%**

> Merging [#12835](https://codecov.io/gh/pydata/pandas/pull/12835?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **+0.07%**

``` diff
@@             master     #12835   diff @@
==========================================
  Files           136        136          
  Lines         49908      49931    +23   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          41876      41962    +86   
+ Misses         8032       7969    -63   
  Partials          0          0          
```
1. File `...ndas/core/groupby.py` (not in diff) was modified. [more](https://codecov.io/gh/pydata/pandas/commit/38a75311c5b138f12bb37d03e04925da964571f1/changes?src=pr#70616E6461732F636F72652F67726F757062792E7079) 
   - Misses `-63` 
   - Partials `0` 
   - Hits `+63`

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