fix POST data not working without string literals

https://curl.se/libcurl/c/curl_easy_setopt.html

Strings passed to libcurl as 'char *' arguments, are copied by the library; the string storage associated to the pointer argument may be discarded or reused after curl_easy_setopt returns. The only exception to this rule is really CURLOPT_POSTFIELDS

curl.bqn

@@ -116,8 +116,8 @@ SetTimeoutms←{timeoutms 𝕊 session:
 # Use a POST method and set data to post
 SetData←{data 𝕊 session:
   "setting up POST request"Check EasySetoptLong⟨session.sessionPtr,curlOptions.post,1⟩
-  "setting POST data"Check EasySetoptStr⟨session.sessionPtr,curlOptions.postfields,data⟩
   "setting POST data size"Check EasySetoptLong⟨session.sessionPtr,curlOptions.postfieldsize,≠data⟩
+  "copying POST data"Check EasySetoptStr⟨session.sessionPtr,curlOptions.copypostfields,data⟩
   session
 }
 

ffi.bqn

@@ -62,6 +62,7 @@ curlOptions←{
 
   # long integer options
   postfieldsize⇐60  # size of the POST input data
+  copypostfields⇐10165 # copy POST input data
   timeout⇐78  # timeout in seconds
   timeoutms⇐155  # timeout in milliseconds
 

tests.bqn

@@ -36,7 +36,7 @@ simpleAPI_testsuite←⟨
   },
   "simple POST with headers"‿{𝕊𝕩:
     headers←⟨"Content-Type: application/json"⟩
-    r←headers Post ⟨httpbinURL∾"/post","{""key"": ""value""}"⟩
+    r←headers Post ⟨httpbinURL∾"/post",¯1000 + 1000 + "{""key"": 1}"⟩
     ! 200=r.code
     ! 0=r.redirectCount
     ! 1=⊑"HTTP/"⍷r.headers