MultiPartParser fails to find the first boundary if the boundary string starts with dashes (e.g., Firefox/Gecko)

## 🐛 Bug Report: MultiPartParser Fails with Dash-Prefixed Boundaries (Firefox Compatibility)

### 📄 Description
The `MultiPartParser` fails to correctly identify the first part of a `multipart/form-data` request when the boundary string in the `Content-Type` header begins with multiple dashes.

This is a common scenario with Firefox (Gecko-based) browsers, which generate boundaries like:

```
----geckoformboundary...
```

---

### ⚙️ Technical Root Cause
In `lib/src/MultiPart.cc`, the method:

```cpp
parse(const HttpRequestPtr &req)
```

extracts the raw boundary string from the header and searches for it directly in the request body.

However, per **RFC 7578**, the actual delimiter in the body is:

```
"--" + boundary
```

#### Example:
- Boundary: `----ABC`
- Actual delimiter: `------ABC`

Because of this:
- The parser searches for `----ABC`
- But the body contains `------ABC`
- The match is found starting at **offset +2** instead of index `0`

This introduces a **2-byte offset error**.

As a result:
- `parseEntity()` begins parsing from the wrong position
- Expected `\r\n` sequences are missed
- Header parsing (e.g., `Content-Disposition`) fails
- The entire multipart parsing fails

---

### 🔁 Steps to Reproduce

Save the following as `repro.cc` and run it:

```cpp
#include
#include
#include

int main() {
// 1. Define a boundary starting with dashes (common in Firefox)
std::string boundary = "----geckoformboundary7805dba873e5a74dd0aa7640be4f989";

// 2. Construct valid RFC 7578 body (Delimiter = "--" + boundary)
std::string body =
"--" + boundary + "\r\n"
"Content-Disposition: form-data; name=\"file\"; filename=\"test.txt\"\r\n"
"\r\n"
"Hello World\r\n"
"--" + boundary + "--\r\n";

auto req = drogon::HttpRequest::newHttpRequest();
req->setMethod(drogon::Post);
req->addHeader("Content-Type", "multipart/form-data; boundary=" + boundary);
req->setBody(std::move(body));

drogon::MultiPartParser parser;
if (parser.parse(req) == 0 && parser.getFiles().size() > 0) {
std::cout << "SUCCESS: Parsed " << parser.getFiles().size() << " files." << std::endl;
} else {
std::cerr << "FAILURE: MultiPartParser could not parse the request." << std::endl;
return 1;
}
return 0;
}
```

---

###Expected Behavior
- The parser should correctly detect the delimiter
- `parser.parse(req)` should return `0`
- Uploaded files should be successfully parsed

---

### Actual Behavior
- The parser returns `-1`
- Multipart data is not parsed due to internal offset mismatch

---

### 💡 Suggested Fix
Ensure the parser searches for the **full delimiter** (`"--" + boundary`) instead of the raw boundary string.

#### Proposed Change (`MultiPart.cc`):

```cpp
// Within MultiPartParser::parse(const HttpRequestPtr &req)
// ...

std::string fixed_boundary = "--" + boundary;
return parse(req, fixed_boundary.data(), fixed_boundary.size());
```

---

### Additional Notes
- This issue primarily affects clients like Firefox that prepend dashes to boundary values.
- The fix aligns the implementation with RFC 7578 and improves compatibility across browsers.