Parameters and Session: allow non-string values

Allow using values that are not strings (i.e. numeric)

Previously, if your $_SESSION contained ints or floats,
is_string($arg) would be false, $value would never be set and the result
would not be what you expected.

Previously:
===========
Session input              output (in errbit)
[                          {
  "thingA" => 10,            "key" => "thingB"
  "thingB" => 7            }
]

Now:
====
Session input              output (in errbit)
[                          {
  "thingA" => 10,            "thingA" => "10",
  "thingB" => 7              "thingB" => "7"
]                          }
Merge pull request #6 from iFixit/non-string-array-values

Parameters and Session: allow non-string values