#### Project Information

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

#### Expected behavior

`account.history()` and `account.get_account_history()` should return only the first blockchain operation of an account when called with `start=0, stop=0, use_block_num=False`.

#### Actual behavior

`account.history()` returns the full history and `account.get_account_history()` returns `limit+1` operations when called with `start=0, stop=0, use_block_num=False`.


#### How to reproduce

```python
from beem.account import Account
import sys
opnum = int(sys.argv[1])
a = Account("stmdev")
ops = list(a.history(opnum, opnum, use_block_num=False))
print("a.history() returned %d ops" % (len(ops)))

ops = list(a.history_reverse(opnum, opnum, use_block_num=False))
print("a.history_reverse() returned %d ops" % (len(ops)))

ops = list(a.get_account_history(index=opnum+1, limit=opnum+1,
                                 start=opnum, stop=opnum,
                                 use_block_num=False))
print("a.get_account_history() returned %d ops" % (len(ops)))
```

Output:
```
# python getop.py 0
a.history() returned 1756 ops
a.history_reverse() returned 1 ops
a.get_account_history() returned 2 ops
```

The same for operation number 1 works:
```
# python getop.py 1
a.history() returned 1 ops
a.history_reverse() returned 1 ops
a.get_account_history() returned 1 ops
```

#### Environment

```
# beempy --version
beempy, version 0.19.34
# python --version
Python 3.6.5
```


### Possible fix

`history()`, `history_reverse()`, and `get_account_history()` check for `if start` / `if stop` at several stages, however this check is `False` for both `None` and `0`. Changing those checks to `if start is not None` / `if stop is not None` could solve this:

```
# python getop.py.py 0
a.history() returned 1 ops
a.history_reverse() returned 1 ops
a.get_account_history() returned 1 ops

# python getop.py.py 1
a.history() returned 1 ops
a.history_reverse() returned 1 ops
a.get_account_history() returned 1 ops
```

A PR has been created for these changes: https://github.com/holgern/beem/pull/19
However, please review these carefully, I'm not sure I covered all cases. The PR includes changes also to `account_history_reverse()`, which does not fail with the test above.


#### GitHub Account
https://github.com/crokkon
A Github issue has been created: https://github.com/holgern/beem/issues/18