variable template fix

[m-mcgowan]

fixes issue #947. Use explicit references to avoid creation of temporaries. Thanks @sergeuz!

A test should comprise an app with variables and functions registered using all the available syntaxes.

---

Doneness:

• Contributor has signed CLA
• Problem and Solution clearly stated
• Code peer reviewed
• API tests compiled
• Run unit/integration/application tests on device
• Add documentation
• Add to CHANGELOG.md after merging (add links to docs and issues)

[m-mcgowan]

Tested with this app:

#include "Particle.h"

String s1 = "A String";
const char* s2 = "a C string";
int i = 42;
bool b = true;
double d = 3.14159;


void setup()
{
   Particle.variable("s1", s1);
   Particle.variable("s2", s2);
   Particle.variable("i", i);
   Particle.variable("b", b);
   Particle.variable("d", d);
}

(Also tested with current 0.5.0-rc.1 and the values are garbage.)

Here's the results:

photon4 [3e0040000e47343432313031] (Photon) is online
  Variables:
    s1 (string)
    s2 (string)
    i (int32)
    b (bool)
    d (double)
photon6 [33002f001147343339383037] (Photon) is offline
pigtail [220038000947333531303339] (Core) is offline
sparkfun_p1 [410035000f47343337373737] (P1) is offline
test [50ff6f065067545646140387] (Core) is offline
Scream:variables mat1$ particle get photon4 s1
A String
Scream:variables mat1$ particle get photon4 s2
a C string
Scream:variables mat1$ particle get photon4 i
42
Scream:variables mat1$ particle get photon4 b
true
Scream:variables mat1$ particle get photon4 d
3.14159

[technobly]

@m-mcgowan looks good! Tested your app on a Photon, wiring/no_fixture tests pass for Photon/Electron U260.