a posting date can cause incorrect stats

[simonmichael]

The posting date causes stats to not count this transaction:

2018/1/1
    (a)   1  ; date:2018/1/2

or

2018/1/1
    (a)  1 ; [2018/1/2]

result:

$ hledger -f a.j stats
Main file                : /Users/simon/src/PLAINTEXTACCOUNTING/hledger/a.j
Included files           : 
Transactions span        : 2018-01-02 to 2018-01-03 (1 days)
Last transaction         : none
Transactions             : 0 (0.0 per day)
Transactions last 30 days: 0 (0.0 per day)
Transactions last 7 days : 0 (0.0 per day)
Payees/descriptions      : 0
Accounts                 : 0 (depth 0)
Commodities              : 0 ()

[simonmichael]

(Should be: Transactions : 1).

Pointers:

• stats command https://github.com/simonmichael/hledger/blob/master/hledger/Hledger/Cli/Commands/Stats.hs#L44
• Posting https://github.com/simonmichael/hledger/blob/master/hledger-lib/Hledger/Data/Types.hs#L201
• Transaction https://github.com/simonmichael/hledger/blob/master/hledger-lib/Hledger/Data/Types.hs#L231