Fix notices caused by Braintree_Instance not having '_attributes' define...

...d (moved from class Braintree_DisbursementDetails #random). Defined Braintree_Error_Validation.__isset() so isset/empty can deal with the private properties. Fixed test Braintree_WebhookNotificationTest::testVerify() that was failing due to poisoned static state.

# Addition of Braintree_Error_Validation.__isset() Behavior Change
## 5.3.10
```
** Old and busted
string(5) "isset"
bool(false)
string(7) "empty()"
bool(true)

** New hotness
string(5) "isset"
bool(true)
string(7) "empty()"
bool(false)
```

## 5.5.9
```
** Old and busted
string(5) "isset"
bool(false)
string(7) "empty()"
bool(true)

** New hotness
string(5) "isset"
bool(true)
string(7) "empty()"
bool(false)
```

# Unit Tests (5.3.10)
## Before
```
PHPUnit 3.7.37 by Sebastian Bergmann.

...............................................................  63 / 155 ( 40%)
..................................................F............ 126 / 155 ( 81%)
.............................

Time: 413 ms, Memory: 12.00Mb

There was 1 failure:

1) Braintree_WebhookNotificationTest::testVerify
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'integration_public_key|c9f15b74b0d98635cd182c51e2703cffa83388c3'
+'integration_public_key|1b0bcd1e59d260f28be85a6239c0dd01120ededc'

/Users/benlake/code/tabletop-initiative/braintree_php/tests/unit/WebhookNotificationTest.php:15

FAILURES!
Tests: 155, Assertions: 343, Failures: 1.
```

## After
```
PHPUnit 3.7.37 by Sebastian Bergmann.

...............................................................  63 / 155 ( 40%)
............................................................... 126 / 155 ( 81%)
.............................

Time: 318 ms, Memory: 12.00Mb
OK (155 tests, 343 assertions)
```

Was this just something missed in a refactor? Very strange for a subclass of Braintree_Instance to have a property only Braintree_Instance used :)