BUG: Styler format `na_rep` arg overwrites existing when not given.

[attack68]

Styler has its own format() method that sets a formatter for columns, and a function set_na_rep() for dealing with missing data.
It seems the two do not play well together:

df = pd.DataFrame([[1,2],[3,4]])
df.iloc[0,0] = np.nan
df.style.set_na_rep('MISS').set_precision(1)

MISS | 2
3.0 | 4

df.style.set_na_rep('MISS').format('{:,.1f}')

nan | 2.0
3.0 | 4.0

df.style.set_na_rep('MISS').format('{:,.1f}', na_rep='PASS')

PASS | 2.0
3.0 | 4.0

could probably do with a review how the attribute na_rep interacts with the formatter and try to make it more dynamic or use pointers instead of static value, if possible.