Error resampling after selecting data
Hi,

I am using pandas version 0.10.1 and numpy version 1.7.1.

I get a missing row if I select a subset of the data before resampling. The following script reproduces the problem with simple data.

``` python
import pandas as pd

from StringIO import StringIO


f = StringIO("""\
time,row_id,a,b
2011-06-03 00:00:05,34083,1.8,1.8
2011-06-03 00:01:05,34083,1.8,1.7
2011-06-03 00:02:05,34083,1.6,1.6
2011-06-03 00:03:05,34083,1.5,1.4
2011-06-03 00:04:05,34083,1.7,1.7
2011-06-03 00:05:05,34083,1.8,1.7
2011-06-03 00:06:05,34083,1.7,1.7
2011-06-03 00:07:05,34083,1.6,1.6
2011-06-03 00:25:39,33909,0.9,0.2
2011-06-03 00:27:39,33909,1.2,1.2
2011-06-03 00:28:39,33909,1.2,1
2011-06-03 00:29:39,33909,1.1,1.1
2011-06-03 00:30:39,33909,1.4,1.4
2011-06-03 00:31:39,33909,1.4,1.3
2011-06-03 00:32:39,33909,1.4,1.3
2011-06-03 00:33:39,33909,1.3,1.3
2011-06-03 00:34:39,33909,1.3,1.3
2011-06-03 00:35:39,33909,1.3,1.2
""")

data = pd.read_csv(f, parse_dates=True, index_col=0)

print "This is the correct resampling result."
print "Notice the starting time is the same as the first line in the sample data"
print data.resample('s').head()
print

print "Now, split the data on each unique row number"
ids = data.row_id.unique()
data_split_by_id = [(row_id, data[data.row_id == row_id]) for row_id in ids]

data = data_split_by_id[0][1]

print "The data still looks as expected"
print data
print

print "But resampling gives the wrong result"
print "Notice that the first line is missing"
print data.resample('s').head()
print

```
