Update content length in POST tests

Upstream curl commit 3aa3cc9b changed MIME multipart boundary from 16
to 22 bytes. Update the tests accordingly. Disable such tests when using
system curl to avoid expected failure.

Author: ecnelises

tests/post.rs

@@ -23,6 +23,7 @@ fn handle() -> Easy {
     e
 }
 
+#[cfg(feature = "static-curl")]
 #[test]
 fn custom() {
     let s = Server::new();
@@ -31,7 +32,7 @@ fn custom() {
          POST / HTTP/1.1\r\n\
          Host: 127.0.0.1:$PORT\r\n\
          Accept: */*\r\n\
-         Content-Length: 142\r\n\
+         Content-Length: 154\r\n\
          Content-Type: multipart/form-data; boundary=--[..]\r\n\
          \r\n\
          --[..]\r\n\
@@ -50,6 +51,7 @@ fn custom() {
     t!(handle.perform());
 }
 
+#[cfg(feature = "static-curl")]
 #[test]
 fn buffer() {
     let s = Server::new();
@@ -58,7 +60,7 @@ fn buffer() {
          POST / HTTP/1.1\r\n\
          Host: 127.0.0.1:$PORT\r\n\
          Accept: */*\r\n\
-         Content-Length: 181\r\n\
+         Content-Length: 193\r\n\
          Content-Type: multipart/form-data; boundary=--[..]\r\n\
          \r\n\
          --[..]\r\n\
@@ -82,6 +84,7 @@ fn buffer() {
     t!(handle.perform());
 }
 
+#[cfg(feature = "static-curl")]
 #[test]
 fn file() {
     let s = Server::new();
@@ -102,7 +105,7 @@ fn file() {
              {}\
              \r\n\
              --[..]\r\n",
-            199 + formdata.len(),
+            211 + formdata.len(),
             formdata
         )
         .as_str(),