Inconsistent behavior using 'in' operator
Operator **in** works differently with str and int in series; please see short snippet below.

`import pandas as pd`
`df1 = pd.DataFrame({'a':['a','b','c'], 'b':['d','e','f']})`
`df2 = pd.DataFrame({'a':[1,2,3], 'b':[3,2,1]})`
`df1.a[0] in df1.a`
False
`df2.a[0] in df2.a`
True
## INSTALLED VERSIONS

commit: None
python: 2.7.5.final.0
python-bits: 64
OS: Darwin
OS-release: 13.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.14.1
nose: None
Cython: None
numpy: 1.6.2
scipy: 0.11.0
statsmodels: None
IPython: 2.1.0
sphinx: None
patsy: None
scikits.timeseries: None
dateutil: 1.5
pytz: 2012d
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.1.1
openpyxl: None
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: 2.5.3 (dt dec pq3 ext)
