text: Title: Timezones in appends are ignored
  Body: #### Arctic Version
  
  ```
  1.62.0
  ```
  
  #### Arctic Store
  
  ```
  VersionStore
  ```
  
  #### Platform and version
  
  Python
  
  #### Description of problem and/or code sample that reproduces the issue
  
  After writing a DataFrame to VersionStore with timestamp index and no timezone, new appends to the same symbol will silently ignore the timezone of the new DataFrame being added. Below is a code example that reproduces the problem:
  
  ```python
  import numpy as np
  import pandas as pd
  
  from arctic import Arctic
  
  store = Arctic('localhost:27017')
  store.initialize_library('mylib')
  mylib = store['mylib']
  
  ts1 = pd.Timestamp('2018-02-02')
  df1 = pd.DataFrame(np.random.randn(1), index=[ts1])
  print 'df1.index.tz:', df1.index.tz
  
  mylib.write('s1', df1)
  
  ts2 = pd.Timestamp('2018-02-03', tz='Europe/London')
  df2 = pd.DataFrame(np.random.randn(1), index=[ts2])
  print 'df2.index.tz:', df2.index.tz
  
  mylib.append('s1', df2)
  
  df3 = mylib.read('s1').data
  print 'df3.index.tz after append:', df3.index.tz
  
  mylib.write('s1', df2)
  
  df3 = mylib.read('s1').data
  print 'df3.index.tz after write:', df3.index.tz
  ```