Fix parsing of MSA Center for PG records

[wilml]

Closes #21

To test,

import arinc424
arinc424.parse("SPACP PGUMPGSRW06LPGPG                0   18018002625                                                                  M   217170804")

Expected results MSA Center has the correct length of 5,

+------------------------+----------+------------------+
| Field                  | Value    | Decoded          |
+------------------------+----------+------------------+
| Record Type            | 'S'      | Standard         |
| Customer / Area Code   | 'PAC'    | Pacific          |
| Section Code           | 'PS'     | Airport MSA      |
| Airport Identifier     | 'PGUM'   | PGUM             |
| ICAO Code              | 'PG'     | PG               |
| MSA Center             | 'RW06L'  | RW06L            |
| ICAO Code (2)          | 'PG'     | PG               |
| Section Code (2)       | 'PG'     | Airport Runway   |
| Multiple Code          | ' '      |                  |
| Continuation Record No | '0'      | Primary Record   |
| Sector Bearing         | '180180' | 180180           |
| Sector Altitude        | '026'    | 26 ft            |
| Sector Radius          | '25'     | 25               |
| Sector Bearing (2)     | '      ' |                  |
| Sector Altitude (2)    | '   '    |                  |
| Sector Radius (2)      | '  '     |                  |
| Sector Bearing (3)     | '      ' |                  |
| Sector Altitude (3)    | '   '    |                  |
| Sector Radius (3)      | '  '     |                  |
| Sector Bearing (4)     | '      ' |                  |
| Sector Altitude (4)    | '   '    |                  |
| Sector Radius (4)      | '  '     |                  |
| Sector Bearing (5)     | '      ' |                  |
| Sector Altitude (5)    | '   '    |                  |
| Sector Radius (5)      | '  '     |                  |
| Sector Bearing (6)     | '      ' |                  |
| Sector Altitude (6)    | '   '    |                  |
| Sector Radius (6)      | '  '     |                  |
| Sector Bearing (7)     | '      ' |                  |
| Sector Altitude (7)    | '   '    |                  |
| Sector Radius (7)      | '  '     |                  |
| Magnetic/True Bearing  | 'M'      | Magnetic         |
| File Record No         | '21717'  | 21717            |
| Cycle Date             | '0804'   | 2008, Release 04 |
+------------------------+----------+------------------+
True