Delete doesn't work for string values

[chrisbartley]

Deletion by importing a value of false doesn't work for string values. For example, import the following:

{"channel_names":["foobar"], "data":[
[1445624602,"this is a string value"],
[]]}

Then call export (here, requesting JSON output) to verify the data:

./bin/export ./data-dev 1 --json feed_34.foobar

Export output should look like this:

{"channel_names":["feed_34.foobar"],"data":[
[1445624602,"this is a string value"]
]}

Then do another import, attempting to delete the value by specifying a false value:

{"channel_names":["foobar"], "data":[
[1445624602,false],
[]]}

But, if you do another export, you'll see that the string value is still there, as above.