get_all_values() skipping columns

Apologies for any mistakes, it's my first time writing an issue on GitHub.

I'm trying to get all the values from a sheet, but when using get_all_values() the returned data is missing certain columns which appear fine when I use .row_values(). I'm not sure if I'm doing something wrong or misunderstanding how get_all_values() works, but after a lot of testing, this feels like a bug to me.

I wrote the code below to compare the retrieved values:
```
row = sheet.row_values(1)
allRow = sheet.get_all_values()[0]
print (row)
print (allRow)
print ("Missing:",list(set(row) - set(allRow)))
```

> ['forwardFacingCName_raw', 'cOrder', 'cNumber', 'mCID', 'dma', 'dmaCode', 'mProName', 'entitlement_raw', 'vMapId', 'productionEntitlement', 'entitlement', 'SAMPLE ZIP', 'PLAYABLE_DMAS', 'forwardFacingCName', 'CID', 'ADD ;', 'ORIGINAL', 'BASIC also in EXTENDED', 'BASIC also in PREMIUM', 'EXTENDED also in PREMIUM', 'SB']

> ['cOrder', 'cNumber', 'mCID', 'dma', 'dmaCode', 'mProName', 'entitlement_raw', 'vMapId', 'productionEntitlement', 'entitlement', 'SAMPLE ZIP', 'PLAYABLE_DMAS']

> Missing: ['ORIGINAL', 'BASIC also in PREMIUM', 'forwardFacingCName_raw', 'EXTENDED also in PREMIUM', 'CID', 'forwardFacingCName', 'ADD ;', 'SB', 'BASIC also in EXTENDED']

### Environment info
Operating System: OSX Sierra 10.12.6
Python version: 3.7.0
gspread version: 3.1.0

Start by reproducing the comparison between sheet.row_values(1) and sheet.get_all_values()[0] shown in the issue, then inspect the get_all_values() and row_values() entry points. Done means get_all_values() returns the same populated columns as row_values() for this sheet, with a regression check for the reported missing values.

Written by the indexing model from the issue text.