bugfix: TokenProvider cache writes

[stobrien89]

Issue #, if available:
Fixes #3175

Description of changes:
Fixes bug where cache() writes an incorrect format.

Smoke and Integ test runs:

vendor/bin/behat --format=progress --suite=smoke --tags='~@noassumerole'
...................................................................... 70
...................................................................... 140
...................................................................... 210
............

111 scenarios (111 passed)
222 steps (222 passed)
0m39.51s (95.43Mb)

vendor/bin/behat --format=progress --tags=integ
...................................................................... 70
...................................................................... 140
...................................................................... 210
......................................

66 scenarios (66 passed)
248 steps (248 passed)
5m37.38s (48.43Mb)

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