fix(beacon): forward Content-Type header from upstream responses (#806)

Add Content-Type to the beacon chain response headers whitelist.
Without this, REST endpoints returning SSZ binary data (e.g.
/eth/v2/debug/beacon/states) are mislabeled as application/json
by dproxy, breaking client-side parsing.

Companion fix: drpcorg/dproxy#2348

src/main/kotlin/io/emeraldpay/dshackle/upstream/beaconchain/BeaconChainSpecific.kt

@@ -123,6 +123,7 @@ object BeaconChainSpecific : AbstractPollChainSpecific() {
     }
 
     override fun getResponseHeadersToForward(): List<String> = listOf(
+        "Content-Type",
         "Eth-Consensus-Version",
         "Eth-Consensus-Finalized",
         "Eth-Execution-Optimistic",