Add an unsafe method to convert CFDictionary to CFMutableDictionary

[jrmuizel]

This change is 

[jdm]

error[E0308]: mismatched types
  --> core-foundation/src/dictionary.rs:65:18
   |
64 |     pub unsafe fn to_mutable(&self) -> CFMutableDictionary<K, V> {
   |                                        ------------------------- expected `dictionary::CFMutableDictionary<K, V>` because of return type
65 |         unsafe { CFDictionary::wrap_under_get_rule(self.0) }
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `dictionary::CFMutableDictionary`, found struct `dictionary::CFDictionary`
   |
   = note: expected type `dictionary::CFMutableDictionary<K, V>`
              found type `dictionary::CFDictionary<_, _>`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0308`.
error: Could not compile `core-foundation`.

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit 87bd8a4 has been approved by jdm

[bors-servo]

⌛ Testing commit 87bd8a4 with merge efb0d28...

[bors-servo]

☀️ Test successful - checks-travis
Approved by: jdm
Pushing efb0d28 to master...