BUG: Mismatch between get and set behavior for slices of floating indices
When _df.index.inferred_type_ is _'floating'_, _df.ix[start:end]_ uses only label-based indexing only when **both** _start_ and _end_ are floating on the ___getitem___ side but when **either** _start_ or _end_ are floating on the ___setitem___ side (where _start_ and _end_ are both either integers or floats within epsilon of integers)

``` python
In [2]: df=p.DataFrame(["a", "b", "c", "d"], index=[1.00, 2.00, 3.00, 4.00])

In [3]: df.ix[1.0:4] # uses positional indexing to get last three rows
Out[3]: 
   0
2  b
3  c
4  d

In [4]: df.ix[1.0:4.0] # uses label-based indexing to get all four rows
Out[4]: 
   0
1  a
2  b
3  c
4  d

In [5]: df.ix[1.0:4] = "y" # uses label-based indexing to set all four rows

In [6]: df
Out[6]:
   0
1  y
2  y
3  y
4  y

In [7]: df.ix[1.0:4.0] = "z" # uses label-based indexing to set all four rows

In [8]: df
Out[8]:
   0
1  z
2  z
3  z
4  z
```
