Perhaps this Default Http Request Headers is expecting the value in the following format:
{
"$type":"SourceCode.SmartObjects.Services.Endpoints.Common.HttpHeader[], SourceCode.SmartObjects.Services.Endpoints.Common, Version=4.0.0.0, Culture=neutral, PublicKeyToken=null",
"$values":[
{
"$type":"SourceCode.SmartObjects.Services.Endpoints.Common.HttpHeader, SourceCode.SmartObjects.Services.Endpoints.Common, Version=4.0.0.0, Culture=neutral, PublicKeyToken=null",
"Name":"Authorization",
"Value":"Bearer Y249emFsaTAzLG91PXVzZXJzLG89ZGF0YTpQQDU1d29yZA=="
}
]
}
Without line-breaks:
{"$type":"SourceCode.SmartObjects.Services.Endpoints.Common.HttpHeader[], SourceCode.SmartObjects.Services.Endpoints.Common, Version=4.0.0.0, Culture=neutral, PublicKeyToken=null","$values":[{"$type":"SourceCode.SmartObjects.Services.Endpoints.Common.HttpHeader, SourceCode.SmartObjects.Services.Endpoints.Common, Version=4.0.0.0, Culture=neutral, PublicKeyToken=null","Name":"Authorization","Value":"Bearer Y249emFsaTAzLG91PXVzZXJzLG89ZGF0YTpQQDU1d29yZA=="}]}
This is an array of HTTP Headers with one for the Authorization Header.
I believe the System Type will also have an 'HTTPHeader' Service Object/SmartObject that can be used to build the string format above; if an array of HTTP Headers are needed.
Based on this post:
http://community.k2.com/t5/K2-blackpearl/unable-to-deserialize-value-httpheader/td-p/97787