Serdes fails to use JSON with Leshan

[dsuni]

Steps to reproduce:

1. Create a standard bootstrap.conf:
   ServerURI=coap://leshan.eclipse.org:5683
   SecurityMode=0
   PublicKey=[PublicKey]
   SecretKey=[SecretKey]
   ServerID=1
   HoldOffTime=30
   ShortServerID=1
   Binding=U
   LifeTime=30
   DefaultMinimumPeriod=1
   DefaultMaximumPeriod=-1
   DisableTimeout=86400
   NotificationStoringWhenDisabledOrOffline=true

2. Run awa_bootstrapd --port 15685 --config bootstrap.conf

3. Run awa_clientd --endPointName somethingrandom --bootstrap coap://127.0.0.1:15685 --verbose

4. Go to http://leshan.eclipse.org/#/clients/somethingrandom

5. Set single value format to JSON

6. Observe some values (doesn't matter which ones)

Expected result:
Operation goes through.

Actual result:
[ERROR] [lwm2m_serdes.c:119] Serialiser not found for type 11543
Leshan receives a 2.00 response, which is not valid.

Reproducible:
Always

Version used:
68cab63 (28 May)