#### Project Information
* Repository: https://github.com/holgern/beem
* Project Name: Beem
* Publisher: @holger80

#### Expected behavior
`account.history_reverse(start=start, stop=stop, only_ops=['comment'])` with `start` and `stop` being `datetime` objects should return the all `comment` operations of an account in the given time range.

#### Actual behavior
`account.history_reverse(start=start, stop=stop, only_ops=['comment'])` may fail with a `TypeError` exception:

```
Traceback (most recent call last):
  File "account_history_reverse.py", line 10, in <module>
    for op in a.history_reverse(start=start, stop=stop, only_ops=['comment']):
  File "/usr/local/lib/python3.6/site-packages/beem/account.py", line 1335, in history_reverse
    while(op_est + est_diff + batch_size < first and block_date < start):
TypeError: '<' not supported between instances of 'str' and 'datetime.datetime'
```

`block_date` is a string in this case and is compared to `start` as a `datetime` object.


#### How to reproduce
```python
#!/usr/bin/python
from beem.account import Account
from datetime import datetime, timedelta
from beem.utils import addTzInfo

a = Account("steemcleaners")
start = addTzInfo(datetime.utcnow()) - timedelta(days=1)
stop = addTzInfo(datetime.utcnow()) - timedelta(days=7)
for op in a.history_reverse(start=start, stop=stop, only_ops=['comment']):
    print(op)
```
![screenshot.png](https://cdn.steemitimages.com/DQmVQDt9GVenf71Kdw3khHgdqtmvSFzMJDz43cPaFeP1Z9C/screenshot.png)

#### Possible fix
Suspicion: `block_date` is converted to datetime objects in other places with `formatTimeString()`. This is not the case for the failing location, `h["timestamp"]` is used directly as a string.

Possible Fix:
```patch
diff --git a/beem/account.py b/beem/account.py
index b41cb58..b4ca42f 100644
--- a/beem/account.py
+++ b/beem/account.py
@@ -1331,13 +1331,13 @@ class Account(BlockchainObject):
             est_diff = 0
             if isinstance(start, (datetime, date, time)):
                 for h in self.get_account_history(op_est, 0):
-                    block_date = h["timestamp"]
+                    block_date = formatTimeString(h["timestamp"])
                 while(op_est + est_diff + batch_size < first and block_date < start):
                     est_diff += batch_size
                     if op_est + est_diff > first:
                         est_diff = first - op_est
                     for h in self.get_account_history(op_est + est_diff, 0):
-                        block_date = h["timestamp"]
+                        block_date = formatTimeString(h["timestamp"])
             else:
                 for h in self.get_account_history(op_est, 0):
                     block_num = h["block"]
```

#### Environment
```
# beempy --version
beempy, version 0.19.32
# python --version
Python 3.6.5
```

#### GitHub Account
https://github.com/crokkon
A Github issue has been created: https://github.com/holgern/beem/issues/13